Four MOSFET full bridge module

ABSTRACT

A molded, leadless packaged semiconductor multichip module includes  100  has four mosfets  10, 12, 14, 16  for a full bridge circuit. The mosfets may include two N-channel and two P-channel devices or four mosfets of the same type, but four N-channel are preferred. In module  100  there are two leadframes  30, 40  for assembling the mosfets. In particular, the two N-channel and two P-channel devices are disposed between two leadframes and encapsulated in an electrically insulating molding compound  84 . The resulting package has four upper heat sinks  44.1 - 44.4  that are exposed in the molding compound  84  for transferring heat from the mosfets to the ambient environment. No wire bonds are required. This can significantly reduce the on resistance, RDS ON . The top or source-drain lead frame  30  may be soldered to the sources and gates of the bridge mosfets.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part application of earlier filedU.S. patent application Ser. No. 12/046,734 filed Mar. 12, 2008, nowU.S. Pat. No. 7,915,721 and is a divisional application of U.S. patentapplication Ser. No. 12/128,130 filed May 28, 2008, now U.S. Pat. No.8,138,585, the specification of which is hereby incorporated in itsentirety.

BACKGROUND OF THE INVENTION

Many devices, such as motors, amplifiers, and the back lights that powerliquid crystal displays are supplied power through an arrangement offour mosfets. The arrangement is generally known as a full bridge or anH-bridge. A control circuit for the full bridge will include anoscillator and pulse width modulator for generating gate controlsignals. The outputs of the control circuit are coupled to gate driversto boost the voltage of the gate control signal to a level acceptable tothe mosfets. The gate drivers may also provide isolation of the mosfetsfrom noise generated by the oscillator in the control circuit.

The control circuit and the gate drivers may themselves be integratedinto a single circuit and packaged in one or more small size packages,such as a molded leadless package. Such packages consume a small areaand a small volume. However, each mosfet is as large as or larger insize than the control circuit. The mosfets can be individually packagedand wired to the control circuit on the printed circuit board of thecontrolled system. However, four separate mosfets consume a lot of areaand volume. As systems shrink, the amount of area and volume for controland mosfet packages is also reduced.

In order to address the problem of fitting more devices intoever-smaller spaces and improving the thermal and electrical performanceof packaged devices, those skilled in the art have packaged four mosfetstogether in a single package. A typical full bridge mosfet package hastwo high side mosfets and two low side mosfets. The four mosfets arearranged in a linear array, one next to the other. A typical four-mosfetpackage is 29×13×3.15 mm, has twenty pins including ten on each of the29 mm sides, and the package configuration is known as a thin shrinksmall outline package (TSSOP) or a small outline integrated circuitSOIC). See FIG. 2. The package has several disadvantages. It is not onlylarge but also has poor thermal/electrical performance and is expensivein its materials, including its lead frame and encapsulating material.

BRIEF SUMMARY OF THE INVENTION

A molded, leadless packaged semiconductor multichip module includes fourmosfets for a full bridge circuit. The mosfets may include two N-channeland two P-channel devices or four mosfets of the same type, but fourN-channel are preferred. At least six examples of bridge modules areincluded. The modules have a smaller footprint (area) than the prior artTSSOP package and a smaller volume.

In some examples, the package has two leadframes for assembling themosfets. In particular, the two N-channel and two P-channel devices aredisposed between two leadframes and encapsulated in an electricallyinsulating molding compound. The resulting package has four upper heatsinks that are exposed in the compound for transferring heat from themosfets to the ambient environment. No wire bonds are required. This cansignificantly reduce the on resistance, RDS_(ON). The top orsource-drain lead frame may be soldered to the sources and gates of thebridge mosfets. An alternate connection technique uses conductive tapeon the sources for attaching the lead frame to the sources and wirebonding for connecting the gates to leadless contacts.

Examples of four same type mosfets include leadframes with two levelsinterconnected by an integral body portion for connecting a source ofone mosfet to a drain of another mosfet. Two such assemblies areincluded in the full bridge module. Ribbon bonds, wire bonds and cliptype bonds connect the sources and gates to leadless contact pads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit schematic of the first example of a four MOSFETcircuit.

FIG. 2 is a prior art TSSOP package.

FIGS. 3 and 4 are top and bottom views of a drain leadframe for thefirst example.

FIGS. 5 and 6 are top and bottom views of the second or source-gate leadframe for the package of the first example.

FIG. 7 is an enlarged plan view of the source-gate lead frame of thefirst example.

FIG. 8 is an end view of the source-gate lead frame of the firstexample.

FIG. 9 is an example of a matrix holding a plurality of drain leadframesfor the first example.

FIG. 10 is a matrix of a plurality of source and gate leadframes for thefirst example.

FIG. 11 is an edge view of FIG. 10.

FIG. 12 shows a plan view of an assembled set of four MOSFETs with drainand source gate leadframes.

FIG. 13 is one end view of the assembly of step 76 in FIG. 15.

FIG. 14 is another end view of the assembly of step 76 in FIG. 15.

FIG. 15 shows a set of steps that includes the assembly andencapsulation of a 4 MOSFET package of the first example.

FIGS. 16-18 show external views of embodiment of the module of the firstexample.

FIGS. 19-22 show external views of a second example.

FIGS. 23 and 24 show top and bottom views of the source-gate lead framefor the second example.

FIG. 25 shows an assembly of the MOSFETs and the leadframes for thesecond example.

FIG. 26 shows a process for assembling and encapsulating four MOSFETsinto a package of the second example.

FIGS. 27-30 show a third example of a four MOSFET package.

FIGS. 31-34 show an alternate version of a four MOSFET package shown inFIGS. 28-30.

FIG. 35 shows a circuit schematic of a four N-channel MOSFET package.

FIG. 36 shows a circuit schematic of a four N-channel MOSFET packagewith internally connected common drains.

FIGS. 37 and 38 show perspective views of a bi-level four MOSFET leadframe and an assembled four MOSFET package.

FIGS. 39 and 40 show external views of the four N-channel MOSFET packageof FIGS. 37 and 38.

FIGS. 41-43 show further details of a four N-channel MOSFET package.

FIG. 44 shows the steps in assembling the fifth example of a fourN-channel MOSFET package.

FIGS. 45 and 46 show perspective drawings of a sixth example of a fourN-channel MOSFET package.

FIGS. 47-50 show internal perspective and external perspective views ofa seventh example of a four N-channel MOSFET package.

DETAILED DESCRIPTION OF THE INVENTION

Turning to FIG. 1, there is shown an electrical schematic for atwo-by-two arrangement of mosfets 10, 12, 14, and 16 disposed inside themodule 100. The example shown in FIG. 1 has two P-channel mosfets 10, 12and two N-channel mosfets 14, 16. However, those skilled in the artunderstand that a full bridge may be made with four N-channel or fourP-channel mosfets. Examples of such bridges are included below. TheP-channel mosfet 10 and the N-channel mosfet 14 are the low side of thebridge. The P-channel mosfet 12 and the N-channel mosfet 16 are the highside of the bridge. The drains of the low side mosfets are common as arethe drains of the high side mosfets. Input signals 21-24 from gatedrivers (not shown) are connected to the leadless gate contacts. Thesources of the P-channel mosfets 10, 12 are connected to leadlesscontacts that are in turn connected to a voltage source, V_(IN). Thesources of the N-channel mosfets 14, 16 are connected to ground. Outputlines 26 and 28 are connected to leadless contacts for powering the backlights of a display (not shown). The operation of the bridge circuit isconventional.

The mosfets 10, 12, 14, 16 are mounted between two lead frames, 30, 40shown in detail in FIGS. 3-6. The following description will refer tothe ‘top’ surface of each lead frame as the surface that makes contactto the mosfet. The ‘bottom’ surface is the surface that is exposed, atleast in part, in the molding compound 84. Drain lead frame 30 has two,spaced apart rectangular members 37, 39. The members 37, 39 receive thedrains of the mosfets on top surface 31 to provide the drain-to-drainconnections between the low side mosfets 10, 14 and the high sidemosfets 12, 16 and no external connections or internal wire bonds areneeded. Surface 31 surface has four die attach pads 32.1-32.4 forholding the dies in place and for making electrical contact to thedrains of the mosfets. The bottom surface 38 of the drain lead frame 30has raised heat sinks 36.1-36.4 that are at least partially exposed inthe encapsulating molding material 84. Openings 33.1-33.4 in the members37, 39 and the space between the members permit molten encapsulatingfluid to flow freely in the mold cavity between the opposite sides ofthe lead frame 30 and thus prevent voids from forming inside thepackage. Severed tie bars 34.1-34.4 and 35.1-35.4 connect the lead frame30 to rails 51.1, 51.2 of the lead frame 50. See FIG. 9.

Source-gate lead frame 40 has three source contact lead frame members42.1-42.3. See FIGS. 5 and 7. Source lead frame member 42.1 is largeenough to contact two sources and provides the common ground connectionfor the N-channel mosfets 14, 16. Source lead frame members 42.2 and42.3 contact the sources of the P-channel mosfets 10, 12. The members42.1-42.3 are spaced apart permit molten encapsulating fluid to flowfreely in the mold cavity between the opposite sides of the lead frame40 and thus prevent voids from forming inside the package. The membersare terminated at one end in leadless contacts and an intermediateportion sloped toward the source contact regions. Surface 48 (FIG. 8)has four heat sinks, including two heat sinks 44.1, 44.4 on member 42.1and one each 42.2, 42.3 on members 41.2 and 41.3. The source-gatecontact pads 43.1-43.4 have leadless contacts at one end, and anintermediate, sloped portion that terminates in a contact pad.

FIG. 9 shows a typical drain lead frame matrix 50 with a plurality ofindividual drain lead frames 30. Tie bars 34, 35 connect the lead frames30 to opposite rails 51.1 and 51.2. The matrix 50 begins as a sheet ofconductive metal, typically copper, that is coated or clad with one ormore layers of nickel, palladium, gold, silver, lead, tin, otherconductive metals and alloys. In a machining operation, the unwantedportions of metal are removed, typically by a stamping die configured toremove the unwanted metal portions from the metal sheet. A conductiveepoxy or a thermal grease holds the heat sinks 36 on the surface 38. Asan alternative, the metal sheet may be forged to flatten selected areasand thereby create raised heat sinks 36 on surface 38.

FIG. 10 shows a typical source-gate lead frame matrix 60 with aplurality of individual lead frames 40. Tie bars 46.1-46.8 and 47.1-47.8(FIG. 7) connect the lead frames 40 to opposite rails 61.1 and 61.2. Thematrix 60 begins as a sheet of conductive metal, typically copper, thatis coated or clad with one or more layers of nickel, palladium, gold,silver, lead, tin, other conductive metals and alloys. In a firstmachining operation, the unwanted portions of metal are removed,typically by a stamping die configured to remove the unwanted metalportions from the metal sheet. After removal, the ends of the gate andsource members are bent at an angle of 45 degrees so that the leadlesscontact ends of the gate and source members are in a plane parallel butspaced from the plane that includes the contacts to the sources andgates. See FIG. 11. A conductive epoxy or a thermal grease holds theheat sinks 44 on the surface 48. As an alternative, the metal sheet maybe forged to flatten selected areas and thereby create raised heat sinks44 on surface 48.

As shown in FIGS. 12, 13 and 14 a full bridge 100 of four packagedmosfets is a smaller package than the prior art of FIG. 2. The multichipmodule 100 is 7×7×0.8 mm. Its area is seven times smaller than the priorart TSSOP package and its volume is twenty-seven times smaller than thevolume of the TSSOP package. The surface area of the module 100 is onlyfourteen percent of the surface area of the prior art TSSOP package. Themultichip module has four mosfets (not shown) arranged in a two by twoarray, in contrast to the linear arrangement of four mosfets in theprior art package. The multichip module 100 has a array of four heatsinks 44.1-44.4 that are attached to the sources of the mosfets forcarrying heat away from the package. See the top view of the module 100in FIG. 16. The heat sinks 44.1-44.4 have their upper surfaces exposedin the electrical insulating encapsulation material 84. Turning to thelower surface view in FIG. 17, the source and gate leadless contacts forthe four mosfets are arranged in two rows 86, 87 along opposite loweredges of the module 100. The module 100 has only twelve leadlesscontacts, compared to the twenty leads required by the prior artpackage. The lower surface has four drain contacts 36.1-36.4 exposed inthe encapsulating material 84. The drain contacts do not requireleadless contacts on the edges. Thus, the wiring of the module to aprinted circuit board is easier because the module 100 requires fewertraces (16 compared to the 20 of the TSSOP).

A process for manufacturing a multichip module 100 is shown in FIG. 15.In a first step 70, a source-gate lead frame 40 is arranged so thatsurface 48 faces up. Next, in step 71 a screen printing operationdeposits soft solder 80 on the upturned surface 48. Mosfets 10, 12, 14,16 are manufactured to have ball grid array contacts to their respectivesources and gates. In step 72 the mosfets are flip chip attached to thesurface 48. A conductive adhesive holds them in place while a reflowoperation melts the soft solder to secure the mosfets to the surface 48.The N-channel mosfet sources are secured to member 42.1 and theP-channel mosfet sources are secured to members 42.2 and 42.3. The gatesof the N-channel mosfets are secured to the gate contact pads 43.1 43.4.The gates of the P-channel mosfets are secured to the gate pads 43.2 and43.3. Step 73 is similar to step 71. Soft solder 83 is applied to theexposed drain surfaces of the mosfets. Then in step 74 the other (drain)lead frame 30 is applied to contact the drains. The drain frame 30 istemporarily held on the drains of the mosfets by a conductive adhesivewhile the solder is reflowed to secure the drains to the frame 30. As aresult, the process provides a dual sided leadframe assembly. Thatassembly along with other assemblies is placed in a cavity of a moldingmachine. Molten electrically insulating resin is transferred under highpressure to fill the cavity mold and encapsulate the dual lead frameassembly. See step 75. The mold cavity, not shown, is formed to haverecesses for receiving and/or sealing the mold cavity off from at leasta portion of the surfaces of the heat sinks. As such, the finishedmodule 100 has exposed portion of the source and drain heat sinks. Themolded assemblies are then separated by singulation from each other toprovide individual modules 100, each containing four mosfets.

One of the features of module 100 is that it has no need for wirebonding. Instead, all of the connections between the leadless contactsand the devices are made using two lead frames. The resulting packageconforms to existing industry standards for size and pin out of moldedleadless packages. The mosfets are fully included inside the moldedmaterial and no portion of any silicon dice is exposed. The module 100has superior thermal performance due to exposed heat sinks and draincontacts on the top and bottom of the package, respectively. Further,the design places the external leadless gate contacts at the ends ofopposite outside edges of the package. Thus, the gates are in thecorners of the package, which is a desired location for assembly of themodule 100 on a printed circuit board.

A second multichip module 200 is shown in FIGS. 19-26. In that example,only the differences between the modules 100 and 200 are illustrated.The process of manufacture is the same. The module 200 reverses therelative positions of the source and gate members. See FIGS. 23 and 24.The gate pads 243.1 and 243.2 are opposite each other on one side of thedevice and the gate pads 241.3 and 241.4 are opposite each other on theopposite side of the device. See FIG. 25. As such the external gateleadless contacts are proximate the middle of the package. The packageis molded to have an external slot 201 between the gate leadlesscontacts. See FIG. 22. The slot provides a space on the printed circuitboard for running gate traces to the leadless gate contacts. See FIGS.20 and 22. The other characteristics of the module 200 are essentiallythe same a module 100. Module 200 has an array of four heat sinksconnected to the source (FIG. 21) and another array of exposed drains(FIG. 20). The drain lead frame 50 is the same in both examples. SeeFIG. 26. Module 200 also has no wire bonding.

A third example of a module 300 is shown in FIGS. 27-30. There the drainleadframes 330 have all of the elements of the leadframe 30 of the priorexamples. In addition, leadless contact pads 343.1-343.8 are alsoincorporated into the leadframe matrix 50. Tie bars (not shown) hold thecontact pads during assembly of the mosfets on the frame 30 and in themold cavity. The contact pads 343.2, 343.3, 343.6 and 343.7 are gatecontact pads. They are connected to the gates of the mosfets by bondwires 312.1-312.4. The wire bonding process is conventional. The mosfetsare attached to the die attach pads of one the drain leadframe 30 bysoft solder, adhesive and then solder reflow. The mosfets 10, 12, 14, 16are assembled onto the frame 340 by applying conductive tape 310.1,310.2, 310.3 to the sources and then source clips 342.1, 342.2 and 342.3are attached to the tape. Then apply heat to cure the conductive tapeand generate a good adhesive bond between the clips and the dice. Thedepending ends of the source clips contact the leadless source contacts343.1, 343.4, 343.5, 343.6. An advantage of module 300 is that the gateleadless contacts are adjacent the slot 301. The conductive tape methoddoes not have the flux problem as in a solder paste and using conductivetape can increase the reliability of the product.

A fourth example 400 of the four mosfet module 400 is shown in FIGS.31-34. The fourth module 400 is similar to module 300 except that thesource and gate leadless contacts are reversed in the respectivepositions. An advantage of module 400 is that the gate leadless contactsare disposed in the corners of the device at the ends of the arrays ofthe source-gate contact pads.

A fifth example 500 of a four mosfet module is shown in FIGS. 35 and37-43. Turning to FIG. 35, there is shown a full bridge circuit withfour N-channel mosfets 510-512. Mosfets 510,511 form the low side of thebridge and mosfets 512,513 form the high side of the bridge. The drainsof mosfets 511, 513 are connected to a power supply 514 and the sourcesof mosfets 510, 512 are separately connected to ground. The sources ofmosfets 511, 513 are connected, respectively, to the drains of mosfets510, 512. Input gate signal lines 520-522 carry gate control signals tothe mosfets 510-513, respectively. Outputs line 526 carries the outputof the low side source-drain node 524 and output line 528 carries theoutput of high side source drain node 527. FIG. 36 shows a module 600where the sources of mosfets 610,612 are internally connected.

In module 500, the full bridge is made using all N-channel mosfets. Thebridge is constructed by using a leadframe whose body connects thesource of one mosfet in half the bridge to the drain of the other mosfetin the half of the bridge. Turning to FIGS. 37, 38, there is shown thetwo-level lead frames 532, 534. Each is the mirror image of the other.Lead frame 534 has a first die attach pad 541 disposed in one plane, asecond die attached pad 543 disposed in a second, parallel plane, and anintermediate member 547 that connects the two die attach pads. See FIG.41. The source of one mosfet 511 will be attached to one die attach pad541 and the drain of the mosfet 510 will be attached to the other dieattach pad 543. Leadless gate contacts 536.1, 536.2 are configured toalso have two planar members and intermediate connecting member toconnect the external leadless contact to internal contact pads thatattach to the gates of the mosfets 510, 511. Likewise, the source ofmosfet 513 will be attached to one die attach pad 540 and the drain ofthe mosfet 512 will be attached to the other die attach pad 544.Leadless gate contacts 538.1, 538.2 are configured to also have twoplanar members and intermediate connecting member to connect theexternal leadless contact to internal contact pads that attach to thegates of the mosfets 512, 513.

The drains of mosfets 511, 513 are electrically and mechanicallyattached to die attach pads 543, 544. Those skilled in the art willappreciate that the source-drain connections are made entirely insidethe package. In addition, the source-drain connections are made withoutbond wires, as is typical of conventional packages. Instead, the sourcedrain connections are made using the bodies of the lead frames forconnecting the source and drain of each half of the bridge to providethe output connections 526,528. This features reduces the complexity ofassembly and save conventional wire bonding steps. Leadless sourcecontacts 537.1, 537.2 receive ribbon bonds 552-555 to connect thesources to the ground. Gates of mosfets 511, 513 are connected toleadless contact pads by wire bonds 556, 557.

Turning to FIGS. 39, 40 there is shown the outside top and bottomappearance of the module 500. The module 500 has a center slot and withfour gate pads, two gate pads at each end of the slot and spaced apartfrom each other by the slot. The two level lead frame and the leadlesscontacts and dies are encapsulated in an electrically insulating moldingcompound 584. Portions of the die attach pads are exposed on the bottomsurface of the molded package to help transfer heat from the module 500to the ambient environment. Exposed low side drain of mosfet 511 andexposed high side drain of mosfet 513 will be connected to the powersupply 504, which is typically a circuit trace on a print circuit board.Leadless contacts 535.1-535.4 and bottom contact pads 526, 528 connectto the internal nodes 524, 527 that correspond to the low sidesource/drain and high side source/drain connections. The source contacts537.1 and 537.2 provide the connections to the ground.

Key steps of the assembly and molding process are shown in FIG. 44. In afirst step 570 solder paste is printed on upturned surfaces 540, 541 oflead frame members 532, 534. Two mosfets with ball grid array contactson their sources and gates are flip chip attached to the upturnedsurfaces. In step 571 the solder is reflowed to attach the mosfets. Instep 572 the lead frame members are turned over and soft solder isapplied to the upturned surfaces of the lead frames (step 573). Twoother mosfets with their drains are attached (step 574) to the softsolder and it is reflowed to fix the mosfets on the lead frames. Thefour gates are wire bonded to the gate leadless contact pads (step 575).In step 576 the sources of the mosfets are ribbon bonded to the leadlesssource contact pads. In a final step 576 the assembly is placed in acavity mold and molten insulating resin is transferred into the cavityto enclose the assembly in an electrically insulating compound.

A sixth example 600 of the four mosfet module is shown in FIGS. 45, 46.Many customer desire the sources be interconnected inside the package.This version of the four mosfet device meets the customer requirementsby using a leadless contact bar 610. The bar has four external leadlesscontact pads and the tops of the pads are integral with a common metalstrip 611.

A seventh example 700 of the four mosfet module is shown in FIGS. 47-50.As in the prior examples, two mosfets are flip chip die attached to onesurface of the double lead frames. The other die attach pad receivesolder paste, have the drains of the third and fourth mosfets attachedto the die pads, and solder is reflowed to fix the die on the pad. Thena conductive epoxy is applied to the source and gate regions. A sourceclip 712 connects the sources to a source bar 722. Gate clips 710, 711connect the gate to leadless gate contact pads.

Those skilled in the art understand that the one or more features of theabove examples may be combined together to comprise further combinationsof lead frames, clips, wire bonding and ribbon bonding.

1. A process for encapsulating multiple dies in an electricallyinsulating compound comprising: providing four mosfets, each mosfethaving a source, gate and drain contacts and arranged as first andsecond pairs of mosfets; directly connecting one source clip to thesource contacts of two of the mosfets, directly connecting a secondsource clip to a source contact of a third mosfet and directlyconnecting a third source clip to a source contact of a fourth mosfet;directly connecting each one of four gate clips to one of the four gatecontacts of the four mosfets, respectively; directly connecting each oneof two drain clips to one of each of the pairs of mosfets, respectively;attaching leadless contacts to the source contacts, gate contacts anddrain contacts; disposing the leadless contacts in a common plane; andencapsulating the mosfets in an electrically insulating compound to forma four-sided package having four edges with two sets of edges oppositeeach other and to expose the drain clips and the leadless contacts. 2.The process of claim 1 comprising the further step of arranging themosfets in a two by two array.
 3. The process of claim 1 comprising thefurther step of arranging the leadless contacts for the gates and thesources in first and second linear arrays on two opposite edges of themodule.
 4. The process of claim 3 comprising the further step ofarranging the gate contacts at the ends of the respective arrays.
 5. Theprocess of claim 3 comprising the further step of arranging the gatecontacts proximate the centers of the respective arrays.
 6. The processof claim 1 wherein the mosfets comprise two n-channel mosfets and twop-channel mosfets.
 7. The method of claim 6 wherein the p-channel drainsare connected to one drain clip and the n-channel drains are connectedto the other drain clip.
 8. The method of claim 7 wherein the sourceclip connected to two mosfets is directly connected to the sourcecontacts of the n-channel mosfets.